For example, in an electrical circuit such as an inverter, load such as an electric motor is driven using a semiconductor switch after AC voltage has been converted to DC voltage by a rectifier and main circuit capacitors. The main circuit capacitors are connected in series to one another between main terminals since the circuit is required to have voltage proof against high DC voltage.
In the above-described inverter, leakage current flows through the main circuit capacitors. Since the leakage current flowing through the main circuit capacitors varies according to individual difference or temperature, voltage sharing of a plurality of main circuit capacitors changes with lapse of time, with the result of possibility that shared voltage would exceed a breakdown voltage of the main circuit capacitors. In view of the possibility, balancing resistances are generally connected between terminals of the main circuit capacitors. Since current constantly flows into the balancing resistances, mere supply of AC voltage to the device always results in a large amount of power loss. In view of this problem, it is desirable to change the number of series-connected smoothing main circuit capacitors after rectification according to AC voltage to be supplied. In this case, the breakdown voltage can be rendered higher when the number of series-connected main circuit capacitors is 3 or above but not 2.
Techniques for obtaining voltage balance have conventionally been proposed. In one proposed technique, transistor-complementary elements are connected to a midpoint of a composition of series-connected electrolytic capacitors and a midpoint of a composition of series-connected resistances is connected to a common base terminal of the transistor-complementary elements.
However, consider now a case where four or more even-numbered compositions of series-connected electrolytic capacitors are used and the adjacent compositions connected in series to each other constitute one pair. In this case, when voltage balance is to be improved between the compositions of each pair, it would be difficult to improve voltage balance between the pairs of compositions.